I. Field of the Invention
The present invention relates generally to electric field sensors and, more particularly, to an electric field sensor with piezoelectric actuated electrodes.
II. Description of Related Art
Low frequency electric field sensors have found numerous industrial, medical, scientific, and military uses. The field sources typically measured in these applications are electrostatically charged particles or surfaces, atmospheric electricity, or active voltage/charge sources such as brainwaves, printed circuit components, and power lines. While there are a variety of means of measuring an ambient electric field, those electric field sensors based on galvanic measurement principles offer the advantage of relative simplicity due primarily to the use of standard metallic conductor electrodes. In the discussion that follows, “galvanic measurement” means that a voltage or current is induced on the electrodes directly by the electric field, and is measured with a sensitive preamplifier.
There are two basic galvanic measurement techniques for E-field measurements in air. High impedance potential gradiometers require special attention to leakage currents and extremely high input impedances. Only recently have mass produced commercially available free space electric potential sensors become available. By comparison, low impedance charge induction (D-Dot) sensors may be constructed from simple and inexpensive off-the-shelf components such as metal plates and operational amplifier integrated circuits.
The low cost and simplicity makes D-Dot sensors attractive, but they have limitations and applications regarding both high sensitivity at low frequencies and small size because their sensitivity is proportional to both the rate of change of the ambient field and the electrode area used to collect the charge. Field mills have employed mechanical chopping of the E-field, typically at tens of hertz, and this modulation permits the ambient DC E-field to be sensed using an induction probe. However, the added size, weight, power, cost, and complexity of bulky drive motors, spinning shafts, and electromagnetic interference shielding of noisy drive components makes these field mills unappealing for small mobile applications or sensor arrays.
MEMS-technology offers the promise of low-cost production due to wafer-level processing. One problem with the previously known traditional MEMS E-field sensors lies in the unwanted interference signals generated by the high voltage electrostatic drive electronics. Thermal drives have been used, but these devices suffer from high power consumption.
A still further disadvantage of the previously known D-Dot sensors is that the output signal from the sensors often includes a high proportion of common mode signal. Thus, it is necessary to account for the common mode signals in order to obtain an accurate measurement of the magnitude of the E-field.